A mmonia entry along surface nephron segments of rats was studied with micropuncture techniques under control and chronic metabolic acidosis conditions. Tubule fluid was collected successively from sites at the end and beginning of the distal tubule and at the end of the proximal tubule of the same nephron. During chronic metabolic acidosis, ammonium excretion doubled. As anticipated, the ammonium concentration (TFNH+4) was significantly higher in proximal tubule fluid during acidosis, and ammonium delivery to end proximal sites increased from 19.4 +/- 2.3 to 34.0 +/- 3.2 pmol/min (P less than 0.001). Although chronic acidosis did not affect TFNH+4 at the beginning of the distal tubule, ammonium delivery to the end of the distal tubule increased from 5.72 +/- 0.97 to 9.88 +/- 0.97 pmol/min. In both control and acidotic groups ammonium delivery was lower (P less than 0.001) to end distal sites than to end proximal sites, indicating net loss in the intervening segment. This loss was greater during chronic metabolic acidosis (23.9 +/- 3.3 vs. 13.6 +/- 2.0 pmol/min in controls, P less than 0.025). In both groups net entry of ammonia, in similar amounts, occurred along the distal tubule (P less than 0.05). In situ pH averaged 6.80 +/- 0.05 at end proximal tubule sites and fell to 6.54 +/- 0.08 at the beginning of the distal tubule (P less than 0.005). Chronic metabolic acidosis did not affect these measurements. The calculated free ammonia at the end of the proximal tubule rose from 9.3 +/- 2.2 to 21 +/- 9 microM (P less than 0.005) during chronic metabolic acidosis, and was also higher at beginning distal sites during acidosis (8.8 +/- 2.4 vs. 2.7 +/- 0.7 microM in controls, P less than 0.05). In both groups ammonia values for the beginning distal tubule fluid were lower than for end proximal tubule fluid. Thus, loss of ammonium in the loop segment is enhanced by chronic metabolic acidosis. Distal entry of ammonia is markedly less than along the proximal tubule and does not change in chronic metabolic acidosis, and ammonia permeabilities for the proximal and distal segments of surface nephrons seem different.